Previous work in our laboratory indicates that spherical cholesterol-rich particles accumulate within human atherosclerotic lesions. Particles accumulate which either stain with filipin (detects unesterified cholesterol) or with oil red O (detects esterified cholesterol). Similar cholesterol-rich particles now have been induced within smooth muscle cells and in association with platelets incubated within plasma- or fibrinogen-derived clots. Incubation of isolated platelets within clots resulted in the accumulation of filipin-positive particles. These platelet-associated particles did not stain with oil red O. When smooth muscle cells were incubated with plasma clots, these cells accumulated intracellular lipid droplets which stained with oil red O but did not stain with filipin. When smooth muscle cells and platelets were incubated together within clots, no particles accumulated in association with platelets. Also, filipin-positive particles rather than oil red O-positive lipid droplets accumulated within the smooth muscle cells. No cholesterol-rich particles accumulated within plasma clots lacking cells. Rare mononuclear white blood cells incubated within clots accumulated cholesterol-rich particles. These findings are relevant to the cholesterol accumulation that occurs within atherosclerotic lesions where cholesterol also deposits within smooth muscle cells and within platelet-containing thrombi. Work will continue during the coming year to determine (using the in vitro systems described above) the source and mechanism of cholesterol accumulation within vascular-associated cells.